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4. How your hear works

Question: When I asked my doctor how my heart works, he said that it would take too long to explain! Can you tell me simply how the normal heart works?

Answer: The healthy normal heart is made up of strong muscle and four valves in full working order. It gets its oxygen from blood supplied by the coronary arteries. It is controlled by an electric circuit which tells it when to beat and how fast to beat. The medical term for contraction of your heart pump (that is, when the heart beats, felt as the pulse) is systole, and when the heart relaxes (between the beats or pulse), diastole.

The nurse at our practice runs a clinic to test blood pressures.

Question: What is blood pressure exactly and why is it so important?

Answer: We all need a blood pressure to keep our blood flowing round our bodies. It is the pressure of the blood in the arteries that is needed for delivering oxygen and food where it is needed and taking away waste products to the kidneys and liver.

Blood pressure has two terms applied to it – systolic and diastolic. The systolic pressure is the top (highest) pressure and is at its maximum following each heartbeat. The bottom (lowest) pressure (diastolic pressure) is the lowest recording between heartbeats when your heart is resting. You might see, for example, 120 (systolic) /80 (diastolic) given as millimetres of mercury and, as mercury’s symbol is Hg, you will see 120/80 mmHg, or 120/80 for short. Mercury is simply a visible liquid used to show the difference between liquid and pressure. When pressure is increased, the mercury is pushed up the scale of the blood pressure machine and readings can be taken in millimetres or mm for short. The mercury sphygmo - manometer is being phased out in some countries and being replaced by electronic machines. This development is regrettable as not all electronic machines are as accurate as the sphygmomanometer (particularly when there is an irregular heart rhythm or very high blood pressure).

Question: I know that the heart beats automatically, but what, if anything, keeps it going?

Answer: The heart gets its instructions rather like an electric circuit. There is a master switch called the sinus node which speeds up the heart rate and slows it down depending on your body’s needs. If you are feeling emotional, or running, it goes faster; if you are resting or sleeping, it goes slower.

The sinus node sends messages to a junction box (the atrio - ventricular node or AV node) which regulates the electric impulses before allowing them through into the ‘wires’ that supply the left heart muscle (ventricle) known as the left bundle, or the right ventricle known as the right bundle. In this way, if some problem develops so that the control switch races away, the junction box protects your heart by not allowing it to go out of control. Normal impulses start in the sinus node, and travel via the atrioventricular node to the ‘bundles of wires’ that supply the left or right ventricle. The right bundle is single, whilst the left bundle divide into branches.



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